(a.) Field of the Invention
This invention relates to improvements in retinal cameras and more particularly to a retinal camera provided with an auxiliary optical system.
(b.) Description of the Prior Art
Some retinal camera is provided with an auxiliary optical system so that not only an image of a retina can be observed or photographed but also the front eye part of an eye to be examined can be observed. The optical system of a conventional retinal camera provided with such auxiliary optical system is as shown in FIG. 1. That is to say, a light from a light source 1 for observation is made to form an image in the position of a light source 3 for photographing by a lens 2, passes through a ring slit 4 arranged near the photographing light source 3 and is made to form an image of the ring slit 4 near an annular mirror 6 by a relay lens 5. This illuminating light is further made to form an image near the pupil 8a of an eye 8 to be examined by an objective 7 and then illuminates a retina 8b. An image of the illuminated retina is formed in the position of an image plane 8'b by the objective 7, passes further through a flare preventing stop 9 provided in the aperture of the annular mirror 6, is reflected by a movable mirror 11, is formed on a reticle 12 by a relay lens 10, is reflected by a prism 13 and is then observed through an eyepiece 14.
In the case of photographing, if the photographing light source 3 is made to project a light, the light will pass through the ring slit 4 and will illuminate the retina the same as in the observation. The same as in the case of the observation, the illuminated retina will be made to form an image by the objective 7 and relay lens 10. However, in this case, if the movable mirror 11 is made to spring up to the position 11' shown by the chain line, the image of the retina will be formed on a film surface 15.
On the other hand, in the case of observing the front eye part of the eye to be examined, an auxiliary lens (positive lens) 16 is inserted between the objective 7 and the image 8'b of the retina by this objective 7 so that an image of the front eye part will be formed in the position of the image 8'b of the retina. When this lens 16 is not inserted, the image of the front eye part will be formed near the stop 9 or on the side nearer to the relay lens 10 but, if the auxiliary lens 16 is inserted, the front eye part will be able to be observed and also photographed the same as when the image of the retina is observed.
In the above explained conventional retinal camera, when the auxiliary lens is inserted to observe the front eye part, the lens system combining the objective and auxiliary lens will not be able to be made high in the magnification. Therefore, the front eye part observed through the eyepiece will not be able to have the details seen. Further, in such optical system, in order that the front eye part may be observed at a high magnification, the position of the principal plane of the auxiliary lens must be brought greatly forward (near to the eye to be examined). However, therefore, the formation and shape of the auxiliary lens will become so unreasonable that it will be difficult to well correct the aberrations. For such reasons, too, it is not desirable to increase the magnification of the lens system formed of the objective and auxiliary lens.
Further, as shown in FIG. 2, the image of the stop 9 by the combined lens system of the objective 7 and auxiliary lens 16 is formed as contracted on the side of the eye to be examined near the objective 7. That is to say, the light incident upon the objective 7 from the front eye part 8c will be limited by the image 9' of the stop and the NA (numerical aperture) will be limited as a result. Therefore, by observing the front eye part, the distance between the eye to be examined and the eye of the observer can be adjusted and the optical axes can be aligned but no detailed observation useful for the diagnosis can be made.